4.2 INFORMATION MODEL
This subsection builds on the concepts presented in section 2 to further describe the types of information that are exchanged and managed within the OAIS. This subsection also defines the specific Information Objects that are used within the OAIS to preserve and access the information entrusted to the Archive. This more detailed model of OAIS-related Information Objects is intended to aid the architect or designer of future OAIS systems. The objects discussed in this subsection are conceptual and should not be taken to imply any specific implementations.
As discussed in section 2, the primary goal of an OAIS is to preserve information for a designated community over an indefinite period of time. In order to preserve this information an OAIS must store significantly more than the contents of the object it is expected to preserve. This subsection analyzes those information requirements used to describe the object classes of data associated with an OAIS. This subsection uses Unified Modeling Language (UML) [D3] object model diagrams to illustrate the concepts discussed in the text. An overview of the notation used and critical object modeling concepts is presented in annex C of this document. An understanding of this notation is required for a full understanding of the concepts presented in this subsection.
Subsection 4.2.1 provides a model of the information required for effective Long Term Preservation of information. Subsection 4.2.2 describes the conceptual objects and containers that represent the contents of an OAIS.
4.2.1 LOGICAL MODEL FOR ARCHIVAL INFORMATION
126.96.36.199 Information Object
A basic concept of the OAIS Reference Model is the concept of information being a combination of Data and Representation Information. The UML diagram in figure 4-10 illustrates this concept. The Information Object is composed of a Data Object that is either physical or digital, and the Representation Information that allows for the full interpretation of the data into meaningful information. This model is valid for all the types of information in an OAIS. Figure 4-10: Information Object
188.8.131.52 Data Object
The Data Object may be expressed as either a physical object (e.g., a moon rock) together with some Representation Information, or it may be expressed as a digital object (i.e., a sequence of bits) together with the Representation Information giving meaning to those bits.
184.108.40.206 Representation Information
The Representation Information accompanying a digital object, or sequence of bits, is used to provide additional meaning. It typically maps the bits into commonly recognized data types such as character, integer, and real and into groups of these data types. It associates these with higher-level meanings: this includes the description of the, possibly complex, ways objects are interrelated (for example, Representation Information could indicate that three numbers represent temperature, latitude and longitude; and they are expressed in degrees Celsius and angular degrees; and they are interrelated in that the temperature is measured at the specified longitude/latitude).
The Representation Information accompanying a physical object like a moon rock may give additional meaning, as a result of some analysis, to the physically observable attributes of the rock. This information may have been developed over time and the results, if provided, would be part of the Information Object.
The remainder of this subsection focuses on the Representation Information object when the Data Object is specialized as a Digital Object.
220.127.116.11.1 Representation Information Types
The Digital Object, as shown in figure 4-10, is itself composed of one or more bit sequences. The purpose of the Representation Information object is to convert the bit sequences into more meaningful information. It does this by describing the format, or data structure concepts, which are to be applied to the bit sequences and that in turn result in more meaningful values such as characters, numbers, pixels, arrays, tables, etc. These common computer data types, aggregations of these data types, and mapping rules which map from the underlying data types to the higher level concepts needed to understand the Digital Object are referred to as the Structure Information of the Representation Information object. These structures are commonly identified by name or by relative position within the associated bit sequences. The Structure Information is often referred to as the ‘format’ of the digital object.
The Representation Information provided by the Structure Information is seldom sufficient. Even in the case where the Digital Object is interpreted as a sequence of text characters, and described as such in the Structure Information, the additional information as to which language was being expressed should be provided. This type of additional required information is referred to as the Semantic Information. When dealing with scientific data, for example, the information in the Semantic Information can be quite varied and complex. It will include special meanings associated with all the elements of the Structural Information, operations that may be performed on each data type, and their inter- relationships. Figure 4-11 emphasizes the fact that Representation Information contains both Structure Information and Semantic Information, although in some implementations the distinction is subjective. It is useful to remember that the Semantic Information associated with parts of some digitally encoded information is independent of the format. For example, the meaning of numbers in a data file is independent of whether they are encoded as scaled integers or as IEEE Reals; the meaning of words in a document is independent of whether the document is Word or PDF.
This figure also shows that Representation Information may contain Other Representation Information. This indicates that the taxonomy of Representation Information presented here is far from complete. For example software, algorithms, encryption, written instructions and many other things may be needed to understand the Content Data Object, all of which therefore would be, by definition, Representation Information, yet would not obviously be either Structure or Semantics. Information defining how the Structure and the Semantic Information relate to each other, or software needed to process a database file would be regarded as Other Representation Information.
Structure Information, Semantic Information and Other Representation Information are both sub-types and components of Representation Information.
Representation Information is an Information Object that may have its own Data Object and its own Representation Information associated with understanding each Data Object, as shown in a compact form by the ‘interpreted using’ association. The resulting set of objects can be referred to as a Representation Network.
As an example, ISO 9660 (reference [D10]) describes text as conforming to the ASCII standard, but it does not actually describe how ASCII is to be implemented. It simply references the ASCII standard which is additional Representation Information that is needed for a full understanding. Therefore the ASCII standard is a part of the Representation Net associated with ISO 9660 and needs to be obtained by the OAIS in some form, or the OAIS needs to track the availability of this standard so that it may take appropriate steps in the future to ensure its ISO 9660 Representation Information is fully understandable.
Figure 4-11: Representation Information Object
18.104.22.168.2 Representation Networks
Representation Information, which is itself an Information Object, may be expressed in physical forms (e.g., a paper document) or in digital forms. When the Representation Information is in digital form, additional Representation Information is needed to understand the bits of the Representation Information as described in the previous subsection. In principle, this recursion continues until physical forms, which can be understood by the Designated Community, are encountered. For example, Representation Information expressed in ASCII needs the additional Representation Information for ASCII, which might be a physical document giving the ASCII standard. Each item of Representation Information can have multiple components, including multiple referenced Representation Information components; each with its own Representation Information.
To preserve the meaning of an Information Object, its Representation Information must also be preserved. This is most easily accomplished when the Representation Information objects are expressed in forms that are easily understandable, such as text descriptions that use widely supported standards such as ASCII characters for electronic versions. One problem with the use of only text descriptions is that such descriptions can be ambiguous. This is addressed by the use of standardized, formal description languages containing well-defined constructs with which to describe data structures. These languages may need to be augmented with text descriptions to convey fully the semantics of the Representation Information.
As the Knowledge Base of the Designated Community changes over time, the Representation Network may need to change accordingly. As noted in 2.2, an OAIS has a choice of whether to collect all the relevant Representation Information or to reference its existence in another trusted or partner OAIS Archive; this is an implementation and organization decision.
The Content Information must be defined and separated into Content Data Object and Representation Information. It is again an implementation and organization decision related to the way Data Objects are ingested and stored in the OAIS. For example, in the case of performing arts, the Content Data Object may be the score as a PDF document, and the Representation Information would include whatever information is needed to re-perform (as the way to use and understand) the piece, such as the way to display the PDF file, the audio processing software needed, placements of hardware such as loudspeakers, movement directions, and a description of how these relate to each other and to the Content Data Object, each of which may be quite complex, encoded in a separate way, and not easily described either simply as Structure or as Semantics. Alternatively, the Content Data Object may be multiple Data Objects including the score, the audio processing software needed, placements of hardware and movement directions. Each of these Data Objects will have its own Representation Information and there will need to be additional Representation Information that describes how the several Data Objects are related.
Two special types of Representation Information are Representation Rendering Software and Access Software. Representation Rendering Software is able to display the Representation Information in understandable forms. For example, the file and directory structure of many CD-ROMs conforms to ISO 9660. This standard is Representation Information describing how most CD-ROM file structures are to be implemented, and it may be obtained as a paper document. However, it may also be obtained as a digital object that needs to be understood as a PDF object. Rather than actually obtaining the documentation of PDF and writing software to understand the ISO 9660 object, an OAIS may use available PDF display software to render the ISO 9660 documentation humanly visible and readable. In this role the PDF display software is referred to as Representation Rendering Software because it is used to render the Representation Information. It also terminates the Representation Network. If the OAIS does not also obtain the associated description of PDF, it needs to record and track this fact because when PDF objects are no longer cost-effective for access and display, the ISO 9660 documentation expressed as a PDF object will need to be migrated to a new form.
Access Software presents some or all of the information content of an Information Object in forms understandable to humans or systems. It may also provide some types of access services, such as displaying, manipulating, processing, or sub-setting, to an Information Object. For some types of Digital Objects, such software may be widely available. It is not necessary for the OAIS to maintain or provide such software. The OAIS may want to maintain and provide this software for more specialized types of Digital Objects.
Since Access software will incorporate some understanding of the Representation Information, some Archives may attempt to use Access Software as a substitute for full Representation Information. Access Software source code, which embodies at least a partial understanding of the associated Representation Information, may be used as documentation expressing such Representation Information. A problem with this approach is that the desired Representation Information may not be clearly identifiable as it may be mixed with various processing and display algorithms, and may be incomplete since the code assumes an underlying operating environment. It may be difficult to tell, from the software code, what Representation Information is missing. The use of Access Software executables, without the source code, such as may occur with proprietary formats, presents a much greater risk for loss of information because it is more difficult to maintain an operating environment for software than to migrate documentation over time. The practical use of emulation techniques to preserve working software is an area of active research. This is a significant issue for those desiring to preserve a look and feel to information access. Migration and software preservation are discussed more fully in section 5.
22.214.171.124 Taxonomy of Information Object Classes Used by OAIS
There are many types of information involved in the Long Term Preservation of information in an OAIS. Each of these types can be viewed as a complete Information Object in that it contains a Data Object and adequate Representation Information to understand the data. This subsection builds on the discussions in 2.2 about the types of supporting information needed to enable Long Term Preservation and the discussion in the previous subsection on the role of Representation Information. The information modeling in this subsection discusses several types of Information Objects that are used in the OAIS. The objects are categorized by their content and function in the operation of an OAIS including Content Information objects, Preservation Description Information objects, Packaging Information objects, and Descriptive Information objects. The following subsections discuss the contents of each of the types of Information Object. Figure 4-12 shows a taxonomy of those Information Objects used within the OAIS. Figure 4-12: Information Object Taxonomy
126.96.36.199.1 Content Information
The Content Information is the set of information that is the original target of preservation by the OAIS. Deciding what the Content Information is may not be obvious and may need to be negotiated with the Producer. The Content Information, which is an Information Object as shown in figure 4-12, is the Content Data Object together with its Representation Information. The Content Data Object in the Content Information may be either a Digital Object or a Physical Object (e.g., a physical sample, microfilm). Any Information Object may serve as Content Information.
The Representation Information for a digital Content Data Object (both semantic and syntactic) is needed to fully transform the bits into the Content Information. In principal, this even extends to the inclusion of definitions (e.g., dictionary and grammar) of any natural language (e.g., English) used in expressing the Content Information. Over long time periods the meaning of natural language expressions can evolve significantly in both general and in specific discipline usage.
As a practical matter, the OAIS needs to have enough Representation Information associated with the bits of the Content Data Object in the Content Information that it feels confident that the members of the Designated Community can enter the Representation Network with enough knowledge to begin accurately interpreting the Representation Information. This is a significant risk area for an OAIS, particularly for those with an expert Designated Community, because jargon and apparently widely understood terms may be short-lived. In such cases extra care needs to be exercised to ensure that the natural evolution of the Designated Community Knowledge Base does not effectively cause information loss from the Content Information.
As described above for an Information Object in general, the Representation Information can also be viewed as being augmented by Access Software that supports the presentation of the Content Information to the Consumer. Examples of this type of software include word processors supporting complex document format representations of Content Information and scientific visualization systems supporting representations of Content Information as a time series or a multidimensional array. Access Software may include rights enforcement tools that allow the access to protected content. The software uses its knowledge of the underlying Representation Information to provide these services.
Often required information will be embedded in the software packages used by the Designated Community to present and analyze the Content Information. A reason for preserving working Access Software arises from a convenience factor. Even with a complete set of Representation Information, practical access to all or part of a digital Content Data Object requires the use of Access Software. Thus a software module that provides useful access to a digital Content Data Object may be preserved in a working state as a matter of convenience.
This is not difficult to do as long as the environment, which supports the software module, is readily available. This environment consists of some underlying hardware and an operating system, various utilities that effectively augment the operating system and storage and display devices and their drivers. A change to any of these may cause the software module to no longer function, to function incorrectly, or to be unable to present results to the application or human user. The complexity of these interactions is what traditionally makes the preservation of working software such an arduous task.
In summary, the use of Access Software to replace Representation Networks is attractive from the point of view of minimizing the resources needed to ingest data and provide current users with access to data. However, the reliance on working software can provide major problems for Long Term Preservation when that software ceases to function. Indefinite Long Term information preservation requires a full and understandable description of the Representation Information. Subsection 5.2 (Preservation of Access and Use Services) discusses some techniques that can be used to preserve software over time and the risks associated with this approach.
An important function of the OAIS is deciding what parts of the Content Information are the Content Data Object and what parts are the Representation Information. This aspect is critical to a clear understanding of what is being preserved. The identification of digital Content Information with its Representation Information objects can be addressed by a series of steps, as follows:
1) Identify the bits comprising the Content Data Object of the Content Information.
2) Identify a Representation Information object that, in some way, addresses all the bits of the Content Data Object and converts them into more meaningful information.
3) For the Representation Information object identified, examine its content to identify if it requires additional Representation Information objects. If it does, obtain the required Representation Information objects. Repeat this step at least until no additional Representation Information objects are identified as required for the Designated Community. 4) Of the Representation Information objects addressed in step 3, for each that is held as a Digital Object, identify any required Representation Information object and repeat steps 3 and 4 until no new Representation Information objects are identified.
5) The Content Information consists of the Content Data Object and each of the Representation Information objects identified in steps 2 through 4.
As an example of this practice, consider an electronic file containing a sequence of values obtained from a sensor looking at the Earth’s environment. There is a second file, encoded using ASCII, which provides information on how to understand the first file. It describes how to interpret the bits of the first file to obtain meaningful numbers. It explains what these numbers mean in terms of the physics of the observation being conducted. It provides the date and time period over which the observations were made, an average value for the observed values, and who made the observations. These two files are submitted to an OAIS for preservation.
Assume that the OAIS determines that the Content Information to be preserved is the observed bits together with their values as numbers and the physical meaning of these numbers. This information is conveyed by the bit sequence within the first file together with the Representation Information from the second file that is needed to transform the first file’s bits into meaningful physical values. Neither the first file’s underlying media nor the particular file system carrying the bits is part of the Content Information in this example. Only part of the second file’s content is considered a part of the Content Information and this is the part that enables the transformation of the bits from the first file into meaningful physical values. In fact this second file does not carry all the Representation Information needed to make this transformation, because the following additional information is needed:
– information that the second file is encoded in ASCII so that it can be read as meaningful characters;
– information on how the characters are used to express the transformations from bits to numbers to meaningful physics values.
This information, typically referred to as a combination of format information and data dictionary information, may also include instrument calibration values and information on how the calibrations are to be applied. All this information may be widely understandable once the ASCII characters are visible because it has all been expressed in English (or some other natural language), or some of it may be in more structured forms that will need additional Representation Information to be understood.
Therefore, the Representation Information of the second file needs additional Representation Information, and this information may need additional Representation Information, etc., forming a linked set of Representations of Representations. This is a good example of the complex Representation Net.
In the example above, there was a determination that the Content Information consisted of the observed sensor values and their meanings. This is by no means the only choice that could have been made. It could just as easily have been decided that the Content Data Object of the desired Content Information was the bit sequences within the first file together with the all the bit sequences within the second file. The fact that some of these latter bit sequences are used to interpret the first file’s bit sequences is just an example of a set of bits that is somewhat self- describing. It is irrelevant that some of the bits in the second file are the basis for information on the date and time period over which the observations were made, the average value for the observed values, and who made the observations. Once it has been determined that all these bits constitute the Content Data Object of the Content Information, then the Representation Information is that information needed to turn them into meaningful information. How extensive this meaning is to be carried and how far the Representation Network needs to be carried are local issues for the OAIS and its related Producer and Consumer communities.
As another example, consider an electronic file containing a word processing document. This binary Data Object will have a complex format that can be seen as a document only after it has been viewed through use of associated Representation Information. In common practice, this viewing will be provided by Access Software that can use internal, or external, Representation Information. The Content Data Object is most likely to be defined as the bit sequence content of the electronic file. The Representation Information is a description of the word processing format, at a minimum, and may include information deemed needed to adequately understand the meaning of the document as viewed. If the word processing format is proprietary, and if adequate Representation Information cannot be acquired which will at the least allow simply viewing, to ensure its Long Term Preservation it may be necessary to migrate the document to another (possibly non-proprietary) format for which Representation Information is more openly available.
As a variation on the above example, it may be decided that the Content Information to be preserved is not the full word processing view of the document, but simply a sequence of text paragraphs that can be adequately represented by ASCII characters. In this case, the OAIS may decide to extract the relevant text characters and save them as a text file. The Content Data Object would be defined, most likely, as the bit stream made up of these characters. The Representation Information would be a description of how to interpret this bit stream as characters, together with any additional information deemed needed to adequately understand the meaning of the text.
188.8.131.52.2 Preservation Description Information
In addition to Content Information, the Archival Information Package must include information that will support the trust in, the access to and context of the Content Information over an indefinite period of time. The specific set of Information Objects, which are required for this function, is collectively called Preservation Description Information (PDI). The PDI must include information that is necessary to adequately preserve the particular Content Information with which it is associated. It is specifically focused on describing the past and present states of the Content Information, ensuring it is uniquely identifiable, and ensuring it has not been unknowingly altered.
This information is typical for all types of Archives and has been classified in the context of traditional Archives. However, the class definitions must be extended for digital Archives.
The following definitions are largely based on the categories discussed in the paper ‘Preserving Digital Information’ (reference [D2]). The relationship between the concepts in OAIS Reference Model and the Preserving Digital Information paper are discussed in annex B of this document. Table 4-1 provides illustrative examples of this information for various popular Content Information types.
– Reference Information identifies, and if necessary describes, one or more mechanisms used to provide assigned identifiers for the Content Information. It also provides those identifiers that allow outside systems to refer, unambiguously, to this particular Content Information. Examples of these systems include taxonomic systems, reference systems and registration systems. In the OAIS Reference Model most if not all of this information is replicated in Package Descriptions, which enable Consumers to access Content Information of interest.
– Context Information documents the relationships of the Content Information to its environment. This includes why the Content Information was created and how it relates to other Content Information objects existing elsewhere.
– Provenance Information documents the history of the Content Information. This tells the origin or source of the Content Information, any changes that may have taken place since it was originated, and who has had custody of it since it was originated, providing an audit trail for the Content Information. This gives future users some assurance as to the likely reliability of the Content Information as it contributes to evidence supporting Authenticity. Provenance can be viewed as a special type of context information.
– Fixity Information provides the Data integrity checks or validation/verification keys used to ensure that the particular Content Information object has not been altered in an undocumented manner. Fixity Information includes special encoding and error detection schemes that are specific to instances of Content Objects. Fixity Information does not include the integrity preserving mechanisms provided by the OAIS underlying services, error protection supplied by the media and device drivers used by Archival Storage. The Fixity Information may specify minimum quality of service requirements for these mechanisms.
– Access Rights Information identifies the access restrictions pertaining to the Content Information, including the legal framework, licensing terms, and access control. It contains the access and distribution conditions stated within the Submission Agreement, related to both preservation (by the OAIS) and final usage (by the Consumer). It also includes the specifications for the application of rights enforcement measures.
These classifications provide a minimum set of PDI; they do not specify a data structure.
Table 4-1: Examples of PDI
The OAIS needs to explicitly decide what the exact definition of Content Information is in order to be able to ensure that it also has the PDI needed to preserve the Content Information. Once the Content Information has been determined, it is possible to assess the Preservation Description Information.
184.108.40.206.3 Packaging Information
The Packaging Information is that information which, either actually or logically, binds or relates the components of the package into an identifiable entity on specific media. For example, if the Content Information and PDI are identified as being the content of specific files in a TAR file, then the Packaging Information may include the name of the TAR file and the fact that it is a TAR file including details of any specific encoding. On the other hand if the Content Information and PDI are files on a CD-ROM, then the Packaging Information may include the ISO 9660 volume/file structure on the CD-ROM. These choices are the subject of local Archive definitions or conventions. The Packaging Information does not necessarily need to be preserved by an OAIS since it does not contribute to the Content Information or the PDI. However, there are cases where the OAIS may be required to reproduce the original submission exactly. In this case the Content Information is defined to include all the bits submitted.
The OAIS should also avoid holding PDI or Content Information only in the naming conventions of directory or file name structures. These structures are most likely to be used as Packaging Information. Packaging Information is not preserved by all Digital Migrations. Any information saved in file names or directory structures may be lost when the Packaging Information is altered. The subject of Packaging Information is an important consideration to the Migration of Information within an OAIS to newer media. This subject is addressed in detail in section 5 of this document.
220.127.116.11.4 Descriptive Information
The Information Objects described previously in this section provide the information necessary to enable the Long Term Preservation function of the Archive. In addition to preserving information, the OAIS must provide adequate features to allow Consumers to locate information of potential interest, analyze that information, and order desired information. This is accomplished through a specialization of the Information Object called Descriptive Information, which contains the data that serves as the input to documents or applications called Access Aids. The Descriptive Information is generally derived from the Content Information and PDI. The Descriptive Information can be viewed as an index to enable efficient access to the associated Information Package via associated Access Aids. Access Aids are documents or applications that can be used to locate, analyze, retrieve, or order information from the OAIS.
4.2.2 LOGICAL MODEL OF INFORMATION IN AN OPEN ARCHIVAL INFORMATION SYSTEM (OAIS)
The previous subsection defines the types of Information Objects that are needed by an OAIS to enable the Long Term Preservation of information and effective access to the preserved information by the Designated Community. This subsection uses those Information Object descriptions to model the conceptual information structures required to accomplish these functions. The models presented in this subsection are not intended to imply an implementation, but rather to highlight the relationship among the types of information needed in the archival process.
18.104.22.168 Information Package
The conceptual structure for supporting Long Term Preservation of information is the Information Package. An Information Package is a container that contains two types of Information Objects, the Content Information and the Preservation Description Information (PDI); the Information Package can be associated with two other types of Information Objects, Packaging Information and Package Descriptions. There are several types of Information Packages that are used within the archival process. These Information Packages may be used to structure and store the OAIS holdings; to transport the required information from the Producer to the OAIS, or to transport requested information between the OAIS and Consumers. There are differing information requirements for each of these functions. The UML diagram in figure 4-13 illustrates the conceptual view of an Information Package. This UML diagram shows that an Information Package contains zero or one Content Information objects, zero or more PDI objects, and is associated with exactly one piece of Packaging Information, which identifies and delimits the Information Package. The Information Package is also associated with one or more Package Descriptions that describe the Content Object to enable efficient access.
Figure 4-13: Information Package Contents
22.214.171.124 Types of Information Packages
There are three subtypes of the Information Package identified in 2.2: the Submission Information Package (SIP), the Archival Information Package (AIP), and the Dissemination Information Package (DIP). The definitions of these package types in section 2 are based on the function of the archival process, which uses the package, and the translation from one package to another as it passes through the archival process. This taxonomy of Information Package types is shown in figure 4-14.
Figure 4-14: Information Package Taxonomy
It is necessary to distinguish between an Information Package that is preserved by an OAIS and the Information Packages that are submitted to, and disseminated from, an OAIS. These variant packages are needed to reflect the reality that some submissions to an OAIS will have insufficient Representation Information or PDI to meet final OAIS preservation requirements. In addition, they may be organized very differently from the way the OAIS organizes the information it is preserving. Finally, the OAIS may provide information to Consumers that does not include all the Representation Information or all the PDI with the associated Content Information being disseminated. These variants are referred to as the Submission Information Package (SIP), the Archival Information Package (AIP), and the Dissemination Information Package (DIP). Although these are all Information Packages, they differ in mandatory content and the multiplicity of the associations among contained classes.
The Submission Information Package (SIP) is that package that is sent to an OAIS by a Producer. Its form and detailed content is typically negotiated between the Producer and the OAIS. Most SIPs will have some Content Information and some PDI, but it may require several SIPs to provide a complete set of Content Information and associated PDI. The Content Information and the PDI both have associated Representation Information, and if there are multiple SIPs involved that use the same Representation Information, it is likely that such Representation Information will only be provided once to the OAIS. As another variation, since some types of PDI will apply to multiple SIPs from the same source, such PDI may be provided in a separate SIP that is without Content Information. The Packaging Information will always be present in some form.
The Descriptive Information associated with a SIP is likely to be provided prior to submitting the SIP to the OAIS, but it may be provided at any time. It may be no more than a text description with a name or title, carried by the Packaging Information, by which the SIP may be recognized.
Within the OAIS, one or more SIPs are transformed into one or more Archival Information Packages (AIPs) for preservation. The AIP has a complete set of PDI for the associated Content Information. The AIP may also contain a collection of other AIPs and this is discussed and modeled later in this subsection. The Packaging Information of the AIP will conform to OAIS internal standards, and it may vary as it is managed by the OAIS. The Descriptive Information associated with an AIP may be extensive and will be managed by the OAIS so that Consumers can find and order the Content Information of interest.
In response to an Order, the OAIS provides all or a part of an AIP to a Consumer in the form of a Dissemination Information Package (DIP). The DIP may also include collections of AIPs, and it may or may not have complete PDI. The Packaging Information will always be present in some form so that the Consumer can clearly distinguish the information ordered. The Packaging Information may take several forms depending on the dissemination media and Consumer requirements. The Descriptive Information associated with a DIP may be provided with the transfer of the DIP, or it may be provided at any time before or after the transfer. Its purpose is to give the Consumer enough information to recognize the DIP from among possible similar packages. It may be no more than a text description with a name or title, as carried by the Packaging Information, by which the DIP may be recognized.
Though the implementation of the AIP may vary from Archive to Archive, the specification of the AIP as a container that contains all the needed information to allow Long Term Preservation and access to Archive holdings remains valid. The information model for the AIP presented in 126.96.36.199 should be used as a reference to establish the types of information required to enable Long Term Preservation and access.
The exact information contents of the SIP and DIP and their relationship to the corresponding AIP are dependent on the agreements between the Archive and its Producers and Consumers. The model for both of these packages is the same as for the Information Package shown in figure 4-13 both in mandatory content and the multiplicity of the associations among contained classes. The subject of transformations between SIP and AIP and between AIP and DIP is further discussed in 4.3.
188.8.131.52 The Archival Information Package
An Archival Information Package (AIP), which is modeled in figure 4-15, is a specialization of the Information Package. The AIP is defined to provide a concise way of referring to a set of information that has, in principle, all the qualities needed for permanent, or indefinite, Long Term Preservation of a designated Information Object. The AIP is itself an Information Object that is a container of other Information Objects. Within the AIP is the designated Information Object, and it is called the Content Information. Figure 4-15: Archival Information Package (AIP)
Also within the AIP is an Information Object called the Preservation Description Information (PDI). The PDI contains additional information about the Content Information and is needed to make the Content Information meaningful for the indefinite Long Term.
The Preservation Description Information requirements in an AIP are much more stringent than the requirements for Preservation Description Information in the general Information Package. While no PDI objects are mandatory in an Information Package, all classes of PDI information must be present in an AIP. This is illustrated in figure 4-16. The contents of each type of PDI are left to the discretion of the individual Archive.
For example, in some OAIS holdings a statement that the creator of the Content Information is unknown may be adequate Provenance Information while in other OAIS holdings it may be mandatory that more complete provenance be researched.
Figure 4-16: Preservation Description Information
The AIP is delimited and identified by the Packaging Information. The Packaging Information may actually be present as a structure on the media that contains the AIP or, it may be virtual in that it is contained in the OAIS Archival Storage function. However, the delimitation and internal identification functions must be well defined in an OAIS.
Each AIP is associated with a structured form of Descriptive Information called the Package Description, which enables the Consumer to locate information of potential interest, analyze that information, and order desired information. The information needed for one Access Aid is called an Associated Description. A single Package Description may contain several Associated Descriptions depending on the number of different Access Aids that can locate, visualize, retrieve or order the associated Content Information and PDI. Figure 4-17 is a UML diagram that models the Package Description and Access Aids.
Figure 4-17: Package Description
The Package Description must contain one Associated Description that supplies data for a Retrieval Aid that allows authorized users to retrieve the Content Information and PDI described by the Package Description. This Retrieval Aid is generally part of the Archival Storage functional area. It translates from the unique identifier assigned by the OAIS to identify the AIP into the set of operations and filenames needed to retrieve the AIP from the file management system used in Archival Storage, and then returns the Content Information and PDI for the requested AIP. In most current Archives, only internal Archive processes and operations personnel and functions are authorized to use this Access Aid. However, as technology advances increase the processing power of the Archive and the bandwidth between the Archive and the user, such access methods as ‘content based queries’ and ‘data mining’ may provide the user with direct read-only access to the Content Information.
The Package Description may also contain any number of Associated Descriptions, each of which contains data for one or more Access Aids. Two additional subtypes of Access Aid are Finding Aid and Ordering Aid.
A Finding Aid is an application that assists the Consumer in locating information of interest. A single AIP may have a number of Associated Descriptions that describe the Content Information using different technologies.
An Ordering Aid is an application that assists the Consumer to discover the cost of and order AIPs of interest. The Ordering Aids also allow users to specify transformations to be applied to the AIPs prior to dissemination. These transformations can include Data Object transformations such as subsetting, subsampling or format transformations. The transformations can also involve modifying the PDI in the AIP prior to dissemination.
The Package Description is not required for the Long Term Preservation of the Content Information but is needed to provide visibility and access into the contents of an Archive. The contents of the Package Description are highly dependent on the structure of the Content Information and PDI it describes. The uses and types of Package Descriptions in an OAIS are further defined in 184.108.40.206.
Figure 4-18 gives a detailed view of the Archival Information Package by expanding the PDI and the Content Information. All the ‘contains’ relationships discussed in this subsection are logical containment relationships. This type of containment relationship may be physical or may be accomplished via a pointer to another object in storage, so an AIP is not necessarily a single file.
Figure 4-18: Archival Information Package (Detailed View)
220.127.116.11 Specialization of the AIP and Package Descriptions
Two specializations of the AIP are discussed in this subsection, the Archival Information Unit (AIU) and the Archive Information Collection (AIC). Figure 4-19 is a UML diagram illustrating this specialization. Both AIU and AIC are subtypes of the AIP and as such contain constructs to enable both Long Term Preservation and Consumer access. The AIU represents the type used for the preservation function of Content Information that is not broken down into other Archival Information Packages. The AIC organizes a set of AIPs (AIUs and other AICs) along a thematic hierarchy, which can support flexible and efficient access by the Consumer community. Conceptually all the AIPs organized by an AIC are contained in the Content Information of that AIC. The difference between AIUs and AICs is the complexity of their Content Information and their associated Package Descriptions and Packaging Information. This reference model considers the differences in the Content Information and associated Packaging and Description functionality between AIU and AIC to be adequately complex and linked to justify the definition of separate classes.
Figure 4-19: Archival Specialization of the AIP From an Access viewpoint, new subsetting and manipulation capabilities are beginning to blur the distinction between AICs and AIUs. Content objects which used to be viewed as atomic can now be viewed as containing a large variation of contents based on the subsetting parameters chosen. In a more extreme example, the Content Information of an AIU may not exist as a physical entity. The Content Information could consist of several input files (or pointers to the AIPs containing these data files) and an algorithm which uses these files to create the Data Object of interest.
From an information preservation viewpoint, the distinction between AIU and AIC remains clear. An AIU is viewed as having a single Content Information object that is described by exactly one set of PDI. An AIC Content Information is viewed as a collection of other AICs and AIUs, each of which has its own PDI. In addition, the AIC has its own PDI that describes the collection criteria and process.
There are two specializations of the Package Description, the Unit Description and the Collection Description. Figure 4-20 is a UML diagram illustrating this specialization. The difference in these two classes is based on the functionality needed to effectively access the contents of an AIU versus the functionality needed to effectively access AIPs that are contained in an AIC. Figure 4-20: Archival Specialization of the Package
To aid in the understanding of these constructs, the next two subsections of this document will use an example of a company setting up an OAIS of digital versions of movies. This example will focus on the information content of constructs in an AIP. Subsection 4.3 illustrates more of the details of the information transformations and data flows in an OAIS.
18.104.22.168 Archival Information Unit
The AIUs can be viewed as the ‘atoms’ of information that the Archive is tasked to store. A single AIU contains exactly one Content Information object (which may consist of multiple files) and exactly one set of PDI. The Archive is free to decide how to construct the AIU and in particular an AIU does not need to be a single file. When an Information Object is ingested into the OAIS a Unit Description, which is a subtype of a Package Description, is created by extracting information from the Content Information and the PDI and adding OAIS-specific information such as a unique identifier. The AIU is illustrated in figure 4-21.
In the example of an OAIS for digital movies, the AIU for a single movie can be viewed as three objects, one containing a digital encoding of the movie in a proprietary format, one containing the Representation Information needed to understand the proprietary format (these two objects form the Content Information), and the other containing facts about the movie such as date of creation, featured actors, director, producer, sequels, movie studio, and a checksum to ensure the integrity of the digital movie (PDI). Since the OAIS reference model is implementation independent, each of these objects could be implemented as one file or multiple files. This type of implementation-dependent information is contained in the Packaging Information. When a movie is ingested into the OAIS a Unit Description for an Ordering Aid can be created by extracting information from the Content Information and the PDI and appending it to the unique ordering information. Figure 4-21: Archival Information Unit (AIU)
22.214.171.124 Unit Description
The Unit Description is a specialization of the Package Description that always contains a set of Associated Descriptions each of which describe the AIU Content Information from the point of view of a single Access Aid. Figure 4-22 is a UML diagram that illustrates the Unit Description contents.
Figure 4-22: Unit Description
All Unit Descriptions must supply an Associated Description for a Retrieval Aid that enables authorized users to retrieve the AIU described by the Unit Description from Archival Storage. This description includes the unique identifier assigned to the AIP by Archival Storage during the Ingest Process.
An important type of Access Aid is the Finding Aid, which is an application that assists the Consumer in locating information of interest. A single AIU may have a number of Associated Descriptions that describe the Content Information using different technologies. Additionally, as new description extraction and display technologies become available, an Archive may want to update the Unit Description associated with each of its AIUs, in order to add a new Associated Description that utilizes the new technology to better describe the AIUs.
In the OAIS for digital movies example, initially, there may be one Associated Description that is a free text description of a movie, another that is a five-minute clip and another that is a row in a relational database that is used by movie collectors to locate movies of interest. After the Archive has been operational for a period of time a technique for supplying compressed digital movies may be developed based on recording every tenth frame. The archivist may decide to create an additional type of Associated Description that is populated using the results of this new technique. If desired, the user can run each of the AIUs contained in the Archive though this compression technique and create a new Associated Description for each movie in the Archive or simply include this Associated Description for new AIUs as they are ingested into the OAIS.
Another important class of Associated Descriptions supplies data for Ordering Aids that allows the Consumer to discover the cost of and order AIUs of interest. The Ordering Aids also allow users to specify transformations to be applied to the AIUs prior to dissemination. These transformations can include Data Object transformations such as subsetting, subsampling or format transformations. The transformations can also involve modifying the PDI in the AIU prior to dissemination.
For example, the OAIS for digital movies could allow a user to order a digital movie as a VHS tape, a laser disc or an MPEG object delivered on-line. Each of these would involve a format transformation and, in theory, an update to the PDI information in the AIP to create accurate PDI for the DIP.
126.96.36.199 Archival Information Collections
The AIU and its associated Unit Description provide the information necessary for a Consumer to locate and order AIUs of interest. However, it can be impossible for a Consumer to sort through the millions of Unit Descriptions contained in a large Archive. This problem is addressed here.
The Content Information of an AIC is composed of complete AIPs each of which have their own Content Information, PDI, and associated Packaging Information and Package Descriptions. These AIPs are then aggregated into Archive Information Collections (AIC) using criteria determined by the archivist. Generally AICs are based on the AIUs of interest having common themes or origins and a common set of Associate Descriptions. At a minimum all OAISes can be viewed has having at least one AIC which contains all the AIPs held by the OAIS.
For example, the OAIS for digital movies may have AICs based on the subject area of the movie such as mystery, science fiction, or horror. In addition the Archive may have AICs based on other factors such as director or lead actor.
A logical model of an AIC is shown in figure 4-23. As in the previous subsections, all of the containment relationships are logical containment and may be physical or may be accomplished via a pointer to another object in storage. For example, the Content Information of an AIC can be created either by creating physical collections of the contained AIPs or by pointing to the contained AIPs. A single AIP can belong to any number of AICs. Figure 4-23: Archive Information Collections Logical View For example, a pattern recognition technique might be created for digital movies and the OAIS for digital movies might offer a service to search its Archives for large structures such as the pyramids or a New York skyline. This type of service is very processing intensive, involving potentially large numbers of AIUs to be transferred from Archival Storage to Access and then running the appropriate process to analyze the Content Information from each AIU. If the results are generally useful, the archivist could summarize the results of this ‘content based query’ into an Associated Description of a new AIC that contains movies with large structures. This technique is frequently referred to as data mining.
An important feature of the AIC, as shown in figure 4-23, is the fact that an AIC is a complete AIP which contains PDI. The PDI provides further information about the AIC such as Provenance on when and why it was created, Context to related AICs, the desired level of security/Fixity and Access Rights Information. This is in addition to the PDI contained in member AIPs. This type of information is often necessary for a Consumer to have confidence in the reliability of an AIC. In the above example, the usefulness of the AIC of movies with large structures is to some extent based on the algorithm used and the Provenance of when the AIC was created or last updated.
188.8.131.52 Collection Descriptions
The Collection Description is a subtype of the Package Description that has added structures to better handle the complex Content Information of an AIC. The Collection Description, which is modeled in figure 4-24, contains the information classes that are contained in the Unit Description.
There are two types of Associated Description in a Collection Description:
– There is one Overview Description that describes the collection as a whole.
– There are zero or more Member Descriptions that separately describe each member of the collection.
Figure 4-24: Collection Description
The required Associated Description in a Collection Description provides information for Ordering Aids that provide a user with access to the entire set of Content Information of the associated AIC and the PDI for the AIC, but not necessarily to the individual AIPs contained in the AIC. The Collection Description may contain the Package Descriptions of the AIPs contained in the AIC. This containment relationship is logical in that the AIC may either include the Package Descriptions of member Information Packages directly or, more commonly, use pointers to the Package Descriptions of the member Information Packages.
This list of the Package Descriptions for contained AIPs in an AIC could provide Access Aids with a method to Retrieve or Order individual members of the AIC.
It also allows alternative concepts for the implementation of Finding Aids that enable the Consumer to locate AIPs of interest that are contained in an AIC. The Associated Descriptions that provide data for these Finding Aids could be implemented either in a centralized fashion searching an Associated Description in the Collection Description or in a distributed fashion by searching the Associated Description of each member Package Description.
Another important benefit of the Collection Descriptions is the ability to define new Access Collections. An Access Collection may be based on new data mining results or it may reflect current phenomena or areas of interest that may not be of permanent interest. Examples of an Access Collection in an OAIS for digital movies might be a new arrivals collection or a ‘twenty most popular titles’ collection that is updated periodically. Another example of an Access Collection is a collection based on the results of a pattern recognition algorithm that has not been verified.
To create an Access Collection, an Archive would create a Collection Description that did not have an associated AIC. The Collection Description could have a customized Associated Member Description that documented the newly mined description data for each member AIP. A specialized finding aid could use this new Associated Member Description in conjunction with existing Associated Descriptions in the Package Description information of each member AIP to locate AIPs of interest to the user. The Package Descriptions of contained AIPs would also supply data for an Ordering Aid, which would allow the Consumer to order the Information Packages of interest to the Consumer.
If an OAIS decides that an Access Collection is valuable enough to be preserved for the Long Term, it can store the required Content Information and PDI in Archival Storage thus creating a new AIC.
Another important application of Access Collections is the concept of locating some members of a collection that have been scheduled for ingest at a future time. In this case, the Associated Descriptions supporting a Finding Aid would allow future AIPs to be located. However, the Associated Description for the Ordering Aid and/or the Retrieval Aid would contain the information that this product was not currently available and allow the user to enter an Event Based Order which would be triggered when the AIP of interest became available.
4.2.3 DATA MANAGEMENT INFORMATION
Currently, Package Descriptions are stored in persistent storage such as database management systems to enable easy, flexible access and update to the contained Associated Descriptions. In addition to the Package Descriptions discussed in the previous subsections, all the information needed for the operation of an Archive could be stored in databases as persistent data classes. Figure 4-25 illustrates the various types of ‘data management information’ within the OAIS. The Archive Administration Information represents the entire range of information required for the day-to-day operation of the Archive. This information includes:
– Policy information which provides pricing information and availability constraints for ordering archived information.
– Request tracking information that records the progress of each user transaction with an Archive. The request tracking process can be very complicated, involving database events and triggers, or as simple as a flat file tracking Order Requests.
– Security information that includes user names and any passwords or other mechanisms needed to authenticate the identity and privileges of Archive users.
– Event Based Order information that provides the information needed to support repeating or future requests.
– Statistical information needed by Archive administration and Management to determine future policies and performance tuning for more effective Archive operation. Examples of these statistics include the number of times an AIP was ordered over a time period and the average time between receiving an order request and shipping the requested holding.
– Preservation process history information that tracks the migrations of AIPs, including media replacements and AIP transformations.
– Customer profile information that enables the Archive to maintain facts such as user name and address to avoid the user’s having to reenter these facts each time he or she enters a request.
– Accounting information that includes the data necessary for the operation of the Archive as a business. The accounting data include payroll data, accounts payable data and accounts receivable data.
These classes are intended as examples rather than an exhaustive list of the data required for Archive administration. These classes are conceptual and individual OAIS implementations may vary significantly. For example, individual OAIS may choose to combine the Customer related information types such as Security and Customer Profile into a single database. Figure 4-25: Data Management Information